The fabrication of micromechanical sensors on microchips is of great interest in many industries because of its potential to reduce the size and cost of devices for sensing environmental and other conditions. An accelerometer is one example of a type of sensor which has wide application possibilities. An accelerometer comprises an acceleration sensing transducer, or sensor, and a sensing circuit for producing an output signal that is representative of the acceleration sensed by the transducer. Ideally, the acceleration sensing transducer and the sensing circuit are fabricated on a single substrate for reasons of size, cost and improved operation. However, due to lack of compatibility between processes for fabricating micromechanical sensors and processes for fabricating semiconductor circuits, the integration of micromechanical sensors and semiconductor circuits into monolithic devices has proven difficult. Methods for fabricating a monolithic chip containing integrated circuitry and a suspended microstructure are disclosed in U.S. Pat. No. 5,326,726 issued Jul. 5, 1994, to Tsang et al.
A micromechanical sensor is typically in the form of a suspended microstructure. In one example of a suspended microstructure, a bridge is suspended above a substrate by four corner anchors. The bridge comprises a central beam having a plurality of movable fingers extending transversely therefrom. A suspended stationary finger is positioned parallel and adjacent to each movable finger of the bridge. The stationary fingers are also suspended on anchors and are cantilevered over the substrate. Typically, the suspended microstructure is embedded in silicon oxide until near the end of the fabrication process. The embedded micromechanical sensor forms a mesa that may have a vertical dimension above the substrate surface of several micrometers.
Because of the relatively high temperatures used in fabricating the micromechanical sensor, it is most practical to fabricate the micromechanical sensor before the semiconductor circuit. However, the presence of the relatively thick micromechanical sensor on the substrate poses problems in fabricating the semiconductor circuit. Typical optical steppers used to pattern circuit features have a depth of focus of about 0.5 micrometers. Thus, differences in height of several micrometers on the substrate surface cause a reduction in resolution. Furthermore, the presence of raised features on the substrate causes difficulty in spinning of photoresists during processing. In general, it is desirable to have a substantially flat upper surface during fabrication of the semiconductor circuit.
A microbridge fabricated in a recess etched in a substrate so that the microbridge is flush with the surface of the substrate is disclosed in U.S. Pat. No. 4,896,098 issued Jan. 23, 1990 to Haritonidis et al. The use of an epitaxial layer to encapsulate a silicon pressure sensor is disclosed by K. Ikeda et al, Sensors and Actuators, A21-A23 (1990), pages 1007-1010 and by K. Ikeda et al, Sensors and Actuators, A21-A23 (1990) pages 146-150.